It is well-known in the art to use power semiconductors of the type specified above for various control purposes, as described in an article by Rüedi, Heinz et al. “Dynamic Gate Controller—A new IGBT gate unit for high current/high voltage IGBT modules”, Power Conversion, June 1995 Proceedings, pages 241 through 249. A method for operating an ignition system and a corresponding ignition circuit is described in an article of Lokuta, Fred et al. “Damit es richtig zündet”, Design & Elektronik, No. 25/26 of Dec. 10, 1996, pages 52 through 54.
A switch-mode power supply of the above-mentioned kind is described in the data sheet “LT1533” from the Linear Technology Company of January 1999.
Power-factor-controllers (PFCs) are well-known in the art in various configurations, for example from an article by Noon, James, “PFC controllers optimised for functional requirements”, PCIM Europe, No. 4, 2000, pages 22 through 25, from another article by the same author “Netzschwankungen korrigieren”, Leistungselektronik & Stromversorgung, April 2000, pages 40 through 43, from an article by Goddard, Thomas, “Controller combines ‘Green’ mode with PFC and PWM”, Power Electronics Engineering Europe, June 1999, pages 12 through 16, as well as from a data sheet “BiMOS PFC/PWM Combination Controller” from the UNITRODE Company of August 1999.
IGBTs and power MOS-FETs are conventional semiconductor components which are distributed from various suppliers, either as single elements or as modules combining a plurality of such IGBTs (cf. data sheet “Neue lötbare Sixpacks der IGBT Plus-Serie Econo Plus” from the Toshiba Company of June 1998).
As already mentioned, such IGBTs and MOS-FETs are typically used for switched-mode power supplies, power-factor-controllers, ignition systems for internal combustion engines and for inverters controlling electrical motors.
For controlling such power semiconductors various driver circuits have been proposed.
German disclosure document DE 34 20 312 C2 discloses a control circuit for a deflection power transistor as used in a deflection circuit of a television set. The control circuit is provided with a sensor arrangement providing an actual signal being proportional to the instantaneous value of the transistor main current. The actual value signal is fed to the control circuit in order to control the switching-on base current to a predetermined value during its entire rise and as a function of the actual value signal.
In a doctoral thesis by Gerster, Christian “Reihenschaltung von Leistungshalbleitern mit steuerseitig geregelter Spannungsverteilung”, published in “Series in Microelectronics”, Vol. 50, Hartung-Gorre Verlag Konstanz, 1995, various methods for controlling the switching behavior of series-connected power semiconductors, for example IGBTs, are described. In this connection reference is made to so-called “Snubber”-circuits, being essentially circuits for limiting a voltage rise.
Further driver circuits for IGBTs are disclosed in articles by Edelmoser, K. H. et al. “Floating, flexible and intelligent gate driver circuit for IGBT half-bridge modules up to 1,200 V, 100 A”, Power Conversion, April 1992 Proceedings, pages 96 through 106, as well as in an article by Bösterling, Werner et al., “Nonproblematic gate drive of IGBT-modules”, Power Conversion, April 1992 Proceedings, pages 87 through 95.
As already mentioned above, IGBTs are today conventionally used in electrical inverters. They generate a frequency variable 3-phase voltage from a DC voltage for operating electrical motors at variable speeds, in particular induction motors, being also designated as “asynchronous motors”. In contrast to thyristors, having been used in earlier times, IGBT transistors may be switched on and off at their gate at any desired moment in time.
In order to effect a switching-on and a switching-off of the IGBTs in a timely controlled manner, the inverters comprise a microcontroller or a control unit. The output signals of these circuits are at 0/5 V or at −5 V/15 V and, therefore, below the control level of commercial IGBTs being at 0/15 V. Further, the output current of these circuits is too low in order to effect a direct coupling to the IGBTs. For these reasons, it is necessary to use a drive circuit. Drive circuits of a type being of interest for the present application generally have a very low impedance output and, therefore, may supply high output currents of the order of several amps. In such a way, commercial IGBT transistors for the above-mentioned applications may be brought into the operational states “ON” and “OFF”.
In order to effect the afore-mentioned switching operations as quickly as possible, the edge steepness of the collector emitter voltage UCE and for the collector current IC should be as high as possible. With short switching-over times low switching-over losses are obtained. On the other hand, the edge steepness of the two afore-mentioned variable quantities may not be selected too high, because in practice there exist limiting side conditions. For example, the variation of the collector-emitter voltage vs. time dUCE/dt must be limited because otherwise inadmissibly high displacement currents appear within the isolation of the motor. The time variation dUCE/dt of the collector-emitter voltage as well as time variation of the collector current dIC/dt must also be limited because otherwise inadmissibly high electromagnetic radiation occurs. Finally, a too high time variation dIC/dt of the collector current in connection with the parasitic inductance could cause an excess voltage at the IGBT.
For these reasons the edge steepness or the time function, respectively, of dUCE/dt and dIC/dt must be held low. On the other hand, this is the phase in which switching losses occur, which, in turn, determine the dissipated heat within the IGBT and, hence, shall also be small.
Prior art drive circuits use passive networks between the driver output and the IGBT gate and the given parameters are tuned to one another. In the above-mentioned article by Bösterling et al. two different approaches are described, one of which (FIG. 5a on page 91) suggesting a fixed resistor within the gate circuit of the IGBT, whereas the other approach (FIG. 6b on page 91) suggests to control the gate of the IGBT via two distinct resistors, each of which being adapted to be switched into the gate circuit via a transistor. Whereas, therefore, in the first mentioned approach one and the same resistor is provided for the switching-on process and for the switching-off process, the second approach utilizes two distinct resistances, wherein the resistor for the switching-off process is generally dimensioned smaller as compared to the resistor for the switching-on process.
This concept, however, has the disadvantage that the resistor or the resistors, respectively, may only be dimensioned according to the power semiconductor data sheet, wherein changes in the operational conditions (e.g. varying temperature), as well as in particular variations in the load may result in partially drastical deteriorations of the operational performance.
The article by McNeill, Neville et al. “Assessment of Off-State Negative Gate Voltage requirements for IGBTs”, IEEE Transactions On Power Electronics, Vol. 13, pages 436 through 440, 1998, comprises a general report about a potential parasitic switching-on of the switched-off IGBT due to a feedback capacity, when the first derivative of the collector-emitter voltage assumes positive values.
An article by Musumeci, S. et al. “A New Adaptive Driving Technique for High Current Gate Controlled Devices”, IEEE Transactions, pages 480 through 486, 1994, discloses a circuit attempting to limit the first derivative of the collector current, in order to limit EMV interferences. For that purpose two switchable voltage sources are used. A close-loop control is not provided.
The prior art drive circuits, therefore, have various disadvantages:
With a fixedly dimensioned passive network it is only possible to determine a function UCE(t) or IC(t) for one specific load. The respective other function IC(t) or UCE(t) follows automatically therefrom. Depending on the particular load of the circuit, the two time functions will vary. Moreover, the two functions are approximately highly non-linear and do not show a constant dUCE/dt and dIC/dt which, however, would be desirable due to the load on the isolation and the electromagnetic radiation.
Furthermore, the edge steepnesses or the time functions dUCE/dt and dIC/dt, respectively, may not be set independently one from another. By means of appropriate networks, however, they may be set differently for the switching-on process and for the switching-off process.
With standard dimensioning of conventional drive networks about 25% to 30% excess voltage appears at the collector-emitter voltage UCE during the switching-off, as compared with the supply voltage. This generally requires a higher voltage class of the IGBT and, therefore, additional costs.
Another disadvantage of prior art drive circuits consists in that the maximum values of dUCE/dt and dIC/dt only appear over a short period of time during the switching process. Considering that the dimensioning is made depending on these threshold values, a higher dissipated power results, as compared with a switching at constant dUCE/dt and dIC/dt.
Finally, conventional drive circuits have the disadvantage that certain components determining the transient behavior are partially located ahead of the gate and partially at the IGBT output. These components are, therefore, exposed to high voltage. This results in high currents, and, therefore, power components at additional costs are required.
The article by Rüedi “Dynamic Gate Controller . . . ”, mentioned at the outset, discloses a topology for an IGBT. A drive circuit comprises a controller for the time function of the collector-emitter voltage UCE as well as the time function of the collector current IC and, finally, a monitoring circuit for the collector-emitter voltage UCE. The signals generated by these modules are combined in a sum node and are used for controlling the IGBT gate. Nothing is disclosed about the exact design, function and cooperation of these modules. The article, further, comprises no evidence about the effectivity of the disclosed method, for example by means of current-voltage curves.
German disclosure document DE 196 10 895 A1 discloses a method for controlling the switching-on process of an IGBT as well as an apparatus for carrying out said method. Although a network is used at the input of the IGBT, the network being designated as a current source, the network is functionally converted into a voltage source by feeding back the input of the network to the inverting input of an output stage (operational amplifier) being arranged ahead of the network. This prior art method also starts from predetermined parameters (characteristic curves) of the IGBT, and the collector current is not measured. As a consequence, the disclosed control is again only valid for one specific point of operation of the IGBT and does not take into account variations in ambient or load conditions.
U.S. Pat. No. 5,390,070 discloses a gated power output stage for inductive loads. The output stage comprises a power semiconductor, wherein both the switched current and the voltage across the power semiconductor are individually detected. From the detected time functions the first derivative is derived wherein the current signal is inverted prior to generating the first derivative. In a sum node the functions corresponding to the first derivatives are superimposed to the control square wave signal for the control electrode of the power semiconductor and the power semiconductor is controlled by means of that superimposed signal. By doing so the high rising and falling speeds, respectively, of the current signal and of the voltage signal shall be reduced.
This prior art power output stage allows an independent setting within the two control circuits of the current signal and the voltage signal, however, no close loop-control starting from a predetermined desired value is used, so that the current function on the one hand and the voltage function on the other hand may not be controlled individually.
Finally European patent specification 493 185 discloses a control circuit for a force commutated power transistor. This circuit corresponds essentially to the approach described in the above-mentioned article by Rüedi.
British disclosure document 2 318 467 A discloses a control circuit for a MOS-FET with inductive load. The circuit is provided with a cross-over switch enabling to connect the gate electrode of the MOS-FET with two different fixed resistors. According to the corresponding description the MOS-FET shall thus be controlled by a signal corresponding to the drain current flowing through the MOS-FET or, alternately, by a signal corresponding to the drain-source voltage across the MOS-FET. Further, it is indicated that both an open-loop control or a close-loop control might be used, however, there is no enabling disclosure how this could be done in practice because the given examples are inoperative and the associated description is incorrect.
U.S. Pat. No. 5,926,012 A discloses a circuit in which the first derivative of the collector current and the first derivative of the collector-emitter voltage of a transistor are detected. These variable quantities are each fed to a comparator which is also supplied with a reference value. The output of the comparators, hence, generate a logical signal “0” or “1”, depending on whether the measured value is above or below the reference value. These digital output signals “0” or “1” are then fed to an open-loop control circuit (FIGS. 6 through 10).
International patent disclosure document WO 00/27032 A discloses a circuit in which certain constant currents are superimposed to the gate current of a MOS-FET. FIG. 3 shows four distinct switches for four distinct current sources by means of which the gate current may be increased or decreased. The switches are operated as a function of certain threshold values so that the gate current is varied stepwise. Therefore, no continuous close-loop control is provided.
It is, therefore, an object underlying the invention to improve a method and an apparatus of the type specified at the outset which avoids the afore-described disadvantages. In particular, the desired switching behavior of the IGBT shall be made possible with the use of components belonging to the lowest possible power class and, hence, the lowest possible costs, by individually controlling the time functions of the collector-emitter voltage dUCE/dt and the collector current dIC/dt. Moreover, the electromagnetic radiation shall be limited to a minimum. Further, the excess voltage during the switching-off shall be reduced to a minimum, so that the admissible voltage range of the power transistor may be used to the widest possible extent. Finally, the overall dissipated power during a switching operation shall be substantially smaller as compared to conventional solutions. Moreover, it shall be possible to monolithically integrate the circuit. Finally, the control stability shall be improved.